This invention relates to multilamp photoflash devices having circuit means for sequentially igniting the flashlamps and, more particularly, to improved means for permitting reliable flashing of an array of photoflash lamps.
Numerous multilamp photoflash arrangements with various types of sequencing circuits have been described in the prior art, particularly in the past few years. Series and parallel-connected lamp arrays have been shown which are sequentially fired by mechanical switching means, simple electrical circuits, switching circuits using the randomly varied resistance characteristics of the lamps, arc gap arrangements, complex digital electronic switching circuits, light-sensitive switching means and heat-sensitive switching devices which involve melting, fusing or chemical reaction in response to the radiant energy output of an adjacently located flashlamp. The present invention is concerned with an improved radiant-energy-activated switching means useful in a relatively inexpensive photoflash unit of the disposable type. In particular, the present switching means is particularly advantageous in photoflash arrays employing lamps adapted to be ignited sequentially by successively applied firing pulses from either a high or low voltage source.
A currently marketed photoflash unit of the high voltage type is described in U.S. Pat. Nos. 3,894,226 and 4,017,728 and referred to as a flip flash. The unit comprises a planar array of high voltage flashlamps mounted on a printed circuit board with an array of respectively associated reflectors disposed therebetween. The circuit board comprises an insulating sheet of plastic having a pattern of conductive circuit traces, including terminal contacts, on one side. The flashlamp leads are electrically connected to the circuit traces, such as by means of eyelets, and the circuitry on the board includes a plurality of solid state switches that chemically change (convert) from a high to low resistance, so as to become electrically conducting after exposure to the radiant heat energy from an ignited flashlamp operatively associated therewith. The purpose of the switches is to promote lamp sequencing and one-at-a-time flashing. One type of solid state switch which operates in this manner is described in U.S. Pat. No. 3,458,270 of Ganser et al, in which the use of silver oxide in a polyvinyl binder is taught as a normally open radiant energy switch. Upon radiant heating, the silver oxide decomposes to give a metallic silver residue which is electrically conductive. Silver carbonate has also been used in lieu of or together with silver oxide. For example, U.S. Pat. No. 3,990,833, Holub et al, describes a mass of a composition comprising silver oxide, a carbon-containing silver salt and a humidity resistant organic polymer binder, the switch mass being deposited on a circuit board so as to interconnect a pair of spaced apart electrical terminals formed by the printed circuitry thereof. A similar type radiation switch exhibiting an even greater humidity resistance at above normal ambient temperatures is described and claimed in U.S. Pat. No. 3,990,832, Smialek et al, which describes the use of a particular stabilizer additive, such as an organic acid, to preclude or reduce the tendancy of the silver source in the switch material from premature conversion to a low electrical resistance when exposed to ambient humidity conditions. U.S. Pat. No. 3,951,582, Holub et al, describes a similar type switch with a colored coating, and U.S. Pat. No. 4,087,233, Shaffer, describes a switch composition comprising silver carbonate, a binder, and an oxidizer such as barium chromate, which is particularly resistant to high relative humidity and above normal ambient temperatures. U.S. Pat. No. 3,969,065, Smialek, describes a solid state switch comprising a mixture of solid copper salt with a humidity resistant organic polymer binder and a finely divided metal reducing agent, and a U.S. Pat. No. 3,969,066, Smialek et al, describes a switch comprising a mixture of finely divided cupric oxide with a humidity resistant organic polymer binder.
In each of the above cases, the switching device comprises a mass of the switch material being interconnected to a pair of spaced apart electrical terminals in the electrical circuit typically comprising a pattern of conductive traces disposed on a dielectric board. A problem has been observed during the functioning of such switch materials, however, in that conversion of the solid state mass from a high to low electrical resistance condition can be sufficiently vigorous that the switch material can be burned off or blown off of the circuit board and thereby fails to provide a low resistance path for the next unflashed lamp. It has been found that this problem can be avoided or reduced considerably by incorporating an electrically nonconductive inert particulate solid, such as glass beads, into the switch composition. For example, a silver carbonate switch composition employing approximately 10% by weight of glass beads to act as a heat sink has been found to provide satisfactory performance in the eight-lamp type of flip flash arrays. The use of a glass bead filler in a solid state switch is also described in U.S. Pat. No. 4,080,155 of Sterling.
More recently, an improved multilamp photoflash unit has been developed which more efficiently utilizes a given housing volume and thereby reduces the cost of the unit per flashlamp contained therein. Such a unit is described in U.S. Pat. Nos. 4,156,269 and 4,164,007. In the particular embodiment described therein, ten lamps are provided in a housing having the same dimensions as the above-discussed eight-lamp flip flash. Such a compact construction results in the lamps being located in closer proximity to the abovementioned solid state radiation switches. In such an application, it has been found that silver carbonate switch compositions including up to 10% by weight of the glass beads do not provide sufficient protection to prevent switch burn off. Further, the filler of glass beads does not significantly lighten the color of the dried switch paste so as to thereby reduce the heat absorbed by the switches. An improved switch composition which avoids these problems is described in copending application Ser. No. 021,398, filed Mar. 19, 1979, and assigned to the present assignee. Avoidance of burn off and reduced heat absorption are attained by replacing part of the silver carbonate and/or silver oxide in the switch composition with a proportion of electrically nonconductive inert particulate solids which comprise as much as 25-65% by weight of the total dried composition. This high proportion of nonconductive inert particulate solids is provided, according to the copending application, by using a filler such as titanium dioxide either alone or in combination with a proportion of glass beads restricted to be not more than 10% by weight of the total dried composition. Limiting the proportion of plain (uncoated) glass beads avoids sodium oxide leaching with resultant electrical leakage. Other inert fillers that can be used are aluminum oxide, aluminum phosphate, barium sulfate, and silicon dioxide. The inert filler acts as a heat absorbing sink and reduces the percentage of the switch which chemically changes when the radiant energy of the lamp heats it up. Further, the inert filler provides a light-colored material composition with random particle shaped so as to reflect and diffuse the radiant energy.
Although the aforementioned switch compositions, especially that of the last-mentioned copending application Ser. No. 021,398, have been found quite satisfactory for use on printed circuit boards having silk-screened circuitry formed of a silver-containing material and operated by a high-voltage pulse source in the order of about 2,000 volts, the electrical resistance across the terminals of such solid state switch masses, after conversion in response to radiation, is generally greater than about two ohms. In the aforementioned U.S. Pat. No. 4,080,155, in column 6, lines 25-27, it is stated that the switches are converted to an electrical resistance less than 100 ohms upon actuation. Although a post-conversion resistance of up to 100 ohms can be quite acceptable for high voltage photoflash arrays, such a "closed" circuit resistance is obviously unacceptable for use in a low voltage photoflash array, e.g., having a battery-operated source generating firing pulses of few volts each. Further, the adherence and mechanical integrity of the converted switch paste of prior art compositions is comparatively poor compared to the improved switch composition to be described hereinafter in accordance with the invention. These qualities of adherence and mechanical integrity pose no problems with the conventionally employed silk-screened, silver-containing circuitry; however, the prior compositions do not consistently provide a good electrical contact after conversion when such solid state switches are employed on circuit boards wherein the metal circuit patterns are provided by certain other techniques, in particular, when using die-stamped aluminum circuitry in order to effect significant cost savings. Printed circuit boards made with die-stamped circuitry are described, for example, in U.S. Pat. No. 3,990,142, Weglin, and photoflash devices including printed circuit boards with die-stamped aluminum circuit patterns are described in copending applications Ser. Nos. 131,614 and 131,711, both filed Mar. 19, 1980 and assigned to the present assignee.